1. Field of the Invention
The invention relates to printers, and more particularly it relates to a device for setting the operating distance or spacing between the printing head and the recording carrier (e.g., paper).
2. Discussion of the Related Art
During initialization of matrix and ink drop (e.g., ink jet) printers, for example, and/or upon insertion of a new recording carrier/substrate (e.g., paper), the printer generally positions or repositions the printing head to within a predetermined distance from the recording carrier or substrate. In order to provide accurate and optimum printing operation, it is important that this predetermined distance fall within a predefined tolerance limit for the specific printer. Should the paper thickness change, for example due to the use of single-ply paper of different thicknesses or the use of multi-ply paper or multiple sheets, the position of the printing head relative to the printing abutment must be reset in order to ensure optimum printing quality and (in the case of matrix printers) a long printing head life.
A distance setting device (i.e., a device for adjusting the position of the print head to account for the thickness of the recording carrier on which printing is to occur) is known from German Patent No. DE 42 21 265 C2. This type of device presupposes a printing head which is freely movable in a direction perpendicular to the recording carrier and includes thickness sensing rollers. An indirectly generated electronic signal (analog signal) emanates from the thickness sensing rollers and is fed to a control unit for the distance setting. In order to produce the signal, a first magnetic element for generating a first magnetic flux is located on the thickness sensing roller. A second magnetic element, generating an opposite second magnetic flux, is located on the printing head carriage. A Hall element is arranged on the printing head carriage in the region of the second magnetic flux, and when a recording carrier is present and the printing head carriage is in a specific lateral position, the drive motor of the distance setting device is activated. The drive motor remains active until the voltage corresponding to an intended or predetermined air gap is applied to the Hall element. This device works relatively accurately, but its accuracy should and can be increased even further. Moreover it is, in part, located in a relatively accessible place on the printing head carriage and is therefore not completely safeguarded from external actions such, for example, as contamination.
Another distance setting device, disclosed in German Patent No. DE 196 11 772 A1, is based on transport rollers which are capable of being resiliently set to the paper thickness and are arranged in pairs on a rigidly mounted driven shaft and on an axis perpendicular to the recording carrier. A measurement signal transmitter arranged on this axis transmits light and cooperates with a measurement signal receiver located on the printing head carriage. Moreover, a position indicator is provided for the printing head carriage. Measurement signals received by the measurement signal receiver are compared with clock pulses of the position indicator, both in the absence of paper and with paper inserted. Based on the resulting difference signal, the printing head distance drive device is operated by a control unit. This type of distance setting device is admittedly cost-effective, because only the measurement signal transmitter and a conventional measurement signal receiver are required. However, it is necessary for the light barrier formed by the light transmitter and light receiver to have highly accurate and stable switch points which cannot be maintained when the ambient and/or operating temperatures for the printer change.
One object on which the invention is based is to eliminate the effect of vibrations caused by stepping motors and the like as they operatively move the carriage supporting the printing head, which often prevent accurate measurement and setting of spacing tolerances between the measuring point and the printing head end face, and thereby to achieve more accurate measurements of the printing substrate thickness and print head position than are attainable in prior art arrangements.
These and other objects are achieved, in accordance with the invention, in a construction in which the printing head is slightly displaceable (deformable) on the supporting carriage as a result of brief contact with the recording carrier which is supported or carried on the printing abutment. The displacement travel (i.e. distance) and/or a displacement angle of the printing head relative to the recording carrier is measured by means of a reflex sensor located in a clearance defined between the carriage and an opposing printing head housing surface. The result of this measurement, in the form of a processed output signal from an evaluation circuit, controls the drive for automatically setting the spacing of the printing head from the recording carrier or from the printing abutment. During brief contact of the printing head with the recording carrier, a slight xe2x80x9cdeformationxe2x80x9d (which amounts to only a few xcexcm) of the printing head on and relative to the carriage occurs and is recorded as a signal change by the reflex sensor without any other disturbing influence. This measuring operation can be triggered automatically and is independent of carriage position.
A reliably determined position of the printing head on the carriage is nevertheless obtained, since the printing head is fastened to the carriage by a spring with a front part facing the carriage and with an elastic intermediate layer interposed between the carriage and print head.
The principle of the reflex sensor of the invention may be employed or implemented in many different embodiments. For example, it is contemplated that the reflex sensor be formed from a pairing arranged on the carriage and printing head housing and implemented by a light emitting diode (LED) and a phototransistor located adjacent the LED and both disposed on a surface of the carriage, and a reflex surface disposed opposite the LED and phototransistor on a surface of the printing head housing.
According to another embodiment, the reflex sensor pairing may be formed by a Hall sensor arranged on the carriage and an opposing magnet fastened to the printing head housing.
Furthermore, the desired reflection of waves or beams can also be achieved by forming the reflex sensor pairing as a magnetically sensitive resistor arranged on the carriage in conjunction with an opposing magnet fastened to the printing head housing.
For reflection of optical beams, it is advantageous if the reflex surface of the reflex sensor is implemented by a mirror. Simple polished metal plates are suitable for use as a mirror, and it is unimportant whether reflection is diffuse or specular.
For all electronic or optical elements operating with waves or beams, the presence of external action or external influences on the waves or beams is harmful to the intended functionality. For this reason, in accordance with this invention the pairing, whether implemented by the light emitting diode/phototransistor and associated reflex surface or the Hall sensor and magnet or the magnetically sensitive resistor and magnet, or any other construction, is disposed in a protected space defined, for example, between an enlarged part of the printing head housing and a surface of the carriage that faces away from the printing abutment.
When the printing head is moved toward the recording carrier or the printing abutment, only a very small electric signal is obtained which would be very difficult, if at all, to process. It is therefore desirable that the phototransistor be capable of recording the resulting change in the signal as the printing head is driven into abutment with and contacts the recording carrier.
In accordance with further features of the invention, the signal is modified in that the signal change is fed in the form of a difference signal to an evaluation circuit in which it is amplified and filtered. In the modified control signal thus obtained at the output of the evaluation circuit, the filtered signal change has been freed of residual interference by an amplifier module using a voltage threshold to define the output signal from the evaluation circuit.
In other embodiments it may be undesirable to position the reflex sensor at the printing head, as when displacement of the printing head is not desired or when a rigid connection of the printing head with the carriage is appropriate to precisely position the printing head on the carriage, as for example with printing heads that are easily exchangeable. In these cases reflex sensor assembly may be arranged and disposed separate from the printing head.
Accordingly, it is also an object of the present invention to generate the measuring signal for the reflex sensor at a location separated or spaced from the printing head without interfering with the ability to obtain a precise measurement and the stable position of the printing head.
This additional object is achieved, in accordance with the present invention, by effecting a brief contact of a resiliently formed element on or portion of the carriage with the recording carrier and/or the printing abutment. This brief contact of the element with the recording carrier and/or printing abutment causes a change in a displacement distance and/or angle that is detected by the reflex sensor. In this alternative embodiment any desired mounting of the printing head can be chosen while maintaining the intended precision of measurement.
The resilient element of the carriage for the reflex sensor measurement may be implemented in many ways. In one advantageous embodiment the resiliently implemented element is formed by a resilient angled carriage portion having a first arm carrying a protrusion disposed confrontingly opposite the recording carrier and printing abutment, and a second arm carrying a part of the reflex sensor. This embodiment is based on ease of manufacturing and provides for the measuring signal to be transferred to the reflex sensor safely and without causing the measuring signal to be transmitted a long distance.
Another embodiment implements the resiliently formed element using a resiliently mounted sensor whose front end forms a protrusion disposed confrontingly the recording carrier and whose rear side is part of the reflex sensor. Since the carriage must be positionable in the direction of the recording carrier and printing abutment, respectively, the sensor will not be in contact with the paper during the time of measurement which lasts only microseconds or less and thus does not interfere with the printing process or paper transport. The positioning of the sensor within a protected space is achieved in these alternate embodiments, in that the reflex sensor is arranged on the carriage within a predetermined slot or space the light emitting diode/phototransistor are disposed at a first side of the slot and the reflex surface is disposed at the opposite side of the slot.
In this second alternate embodiment the resilient element is bolt-shaped and is provided at its distal end with the reflex surface. In this manner, the reflex surface moves with the element, while the light emitting diode and phototransistor are rigidly carried on the carriage.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.